Diphenhydramine

Question 1

What is the trade name for diphenhydramine?

Answer 1

Benadryl

Question 2

What drug class is diphenhydramine in?

Answer 2

First generation antihistamine/ H1-receptor antagonist

Question 3

What are the clinical uses of diphenhydramine?

Answer 3

• Prevent or treat allergic/hypersensitivity reactions to other drugs, blood products, or common allergens; given along with epinephrine in anaphylaxis
• Treatment of motion sickness, parkinsonism, sedative/mild hypnotic, antiemetic
• antipruritic, antitussive, anticholinergic, antinausea and topical anesthetic properties
• treatment of antipsychotic induced extrapyramidal symptoms

Question 4

What is diphenhydramine usually given in combination with for prophylaxis?

Answer 4

Cimetidine (H2-antagonist) and steroids for prophylaxis in patients with chronic atopy (hereditary predisposition to developing allergies), latex allergy, and before chemonucleolysis and dye studies which are known to be associated with allergic reactions

Question 5

What is the MOA of diphenhydramine?

Answer 5

-Blocks Histamine-1 receptor by competing with histamine on the effector cell receptor sites, thus preventing the effects of histamine peripherally

Question 6

Where are the effector cells (that have H1 receptors) for Diphenhydramine?

Answer 6

effector cells for this medication are in the GI tract, blood vessels, and respiratory tract

Question 7

What is the onset and duration of action of diphenhydramine?

Answer 7

Onset: RAPID
DOA: 4-6hours

Question 8

What is the half-life of diphenhydramine? what is different for the elderly?

Answer 8

• 2-8hrs

- 13.5 hours in the elderly

Question 9

How is diphenhydramine metabolized?

Answer 9

Extensive hepatic metabolism with smaller degrees in the pulmonary and renal systems; SIGNIFICANT first-pass effect

Question 10

What is the elimination route for Diphenhydramine?

Answer 10

A portion of the drug is excreted unchanged in the urine

not much info on extensive pharmacokinetics

Question 11

What is the Volume of Distribution of diphenhydramine?

Answer 11

widely distributed in the body, and its distribution includes the CNS

Question 12

Is diphenhydramine protein bound, if so how much?

Answer 12

78% protein bound

Question 13

What is the Bioavailability of oral diphenhydramine?

Answer 13

40-60%

Question 14

What are the side effects of diphenhydramine?

Answer 14

• moderate somnolence, drying of nasal mucosa, dry mouth
• Hypotension, dizziness, tachycardia, urinary retention, seizures, epigastric distress, thickening of bronchial secretions, irritability, constipation
• nervousness, fatigue, photosensitivity, N/V, wt gain, anorexia, hemolytic anemia, thrombocytopenia, agranulocytosis, tremor, palpitations, paresthesia, blurred vision, disturbed coordination, paradoxical excitement, insomnia, euphoria, confusion, rash, angioedema, diarrhea, urinary frequency
• seizures

Question 15

What is different regarding adults and pediatric populations vs young pediatric populations as side effects of diphenhydramine? what about elderly patients?

Answer 15

• in adults and pediatric patients, this drug can cause decreased alertness, while in YOUNG pediatric patients it may produce excitation
• elderly patients more likely to cause dizziness, sedation, hypotension

Question 16

What can diphenhydramine administration do with your anesthesia?

Answer 16

Due to sedative side effects, administration immediately before end of anesthesia may delay recovery

Question 17

What are the contraindications to diphenhydramine?

Answer 17

• all first generation H1 receptor antagonists are contraindicated for use when DRIVING or operating heavy machinery
• This drug is contraindicated in premature infants, neonates, or nursing mothers
• dont use as local anesthetic
• dont give to ppl with known hypersensitivity reaction

Question 18

Why should you NOT use diphenhydramine as a local anesthetic?

Answer 18

due to the risk of local necrosis!

Question 19

When should you use CAUTION with diphenhydramine?

Answer 19

-use with caution in pts with lower respiratory disease, increased IOP, hyperthyroidism, CV disease, hypertension, narrow-angle glaucoma, stenosing peptic ulcer, pyloroduodenal obstruction, symptomatic prostatic hypertrophy, or bladder neck obstruction

Question 20

What does diphenhydramine do to the hepatic enzymes? Which enzymes?

Answer 20

Moderately inhibits CYP2D6!!!!!

Question 21

What drugs does diphenhydramine have additive effects with?

Answer 21

Alcohol and other CNS depressants

Question 22

What drug can intensify and prolong the anticholinergic effects of diphenhydramine?

Answer 22

MAOIs

Question 23

Co-administration with any of these drugs (3) can cause anticholinergic syndrome

Answer 23

Amantadine
Rimantadine
Anticholinergic drugs

Question 24

What can diphenhydramine cause with beta blockers (such as atenolol)

Answer 24

increase the bioavailability of atenolol

Question 25

What do cholinergic agents do to the effects of Diphenhydramine?

Answer 25

(includes donepezil, rivastigmine, and tacrine) cholinergic agents antagonize the therapeutic effects

Question 26

What can diphenhydramine do to Digoxin and/or levodopa?

Answer 26

May decrease gastric degradation and increase the amount of digoxin and levodopa absorbed by delaying gastric emptying!

Question 27

What does administration with diphenhydramine and neuroleptics do?

Answer 27

therapeutic effects may be antagonized

Question 28

What herbs/nutraceuticals should you avoid with diphenhydramine? why?

Answer 28

Valerian, St Johns Wort, Kava Kava, Gotu Kola

-may increase CNS depression

Question 29

What is the IV dosage of diphenhydramine?

Answer 29

10-50mg IV q6-8hrs

Question 30

What is the oral dosage of diphenhydramine?

Answer 30

25-50mg oral q6-8hrs

Question 31

What is the available IV concentration of diphenhydramine?

Answer 31

IV form is available in concentration of 50mg/mL with a pH adjusted to a range of 5-6